The forward facing turn signal lamps of automotive vehicles have two separate and distinct aspects. A first aspect is purely functional; that is, the lamp must provide sufficient illumination for its intended purpose under both normal and adverse conditions. Standards for such conditions are generally set by one or more governmental agencies and are adhered to by automotive designers and lamp manufacturers.
The second aspect is that of appearance; that is, the lamp unit must not only be functional but aesthetically pleasing to designers and customers alike.
Front or forward lighting signal lamps have to endure environmental temperatures of up to 105 degrees centigrade. If they are positioned close to the headlamps, the photometric requirements can be up to 2.5 times higher than for lamps positioned further away. For many years the practice has been to employ a conventional incandescent lamp for this purpose and, to distinguish the signal lamp from the normal vehicle headlamps, to use a colored lens in front of the lamp, usually yellow. When the automotive industry decided, for aesthetic reasons, to use clear lenses, it was necessary to develop lamps that emitted in the yellow region of the visible spectrum. This was accomplished by applying a coating to conventional P21 and S8 incandescent lamps, the coating reflecting blue and green light but transmitting yellow light. Such a dichroic interference coating applied to incandescent lamps are shown in European laid-open specification EP 0 986 093 A1 (Bodmer). Dichroic coatings are those coatings which will selectively transmit some wavelengths while reflecting others. Commercial embodiments of such coatings are sold by Osram or Osram Sylvania with the trademark “Diadem” for lamps with an interference coating having an opal appearance when unlit and an amber appearance when lit. A coating on an incandescent bulb that transmits red light while reflecting blue and green light is disclosed in U.S. Pat. No. 6,661,164 (Ruemmelin).
Other bulb coatings are shown in U.S. Pat. Nos. 7,176,606 (Schaefer), and 5,200,855 (Meredith). U.S. Pat. No. 6,369,510 B1 (Shaw) teaches a blue tinted lamp coating on a lamp capsule. U.S. Pat. No. 4,839,553 (Mellor) discloses an arrangement of a reflector lamp and curved lamp both with dichroic coatings
Light emitting diodes (LEDs) have been suggested as possible alternatives for the incandescent lamps and yellow emitting LEDs (comprised of AlInGa) do exist; however, these materials exhibit a strong decrease in light output with increasing temperatures. At the postulated 105 degree centigrade environmental temperatures encountered near the front lights of vehicles, these LEDs cannot survive without displaying a strong thermal roll-off of 40% or more.
White-light emitting LEDs are available; however, the use of white-emitting LEDs for turn signal applications would require the use of a yellow filter, resulting in the very appearance (a yellow unlit look) that manufacturers do not want. Such a technique is shown in U.S Published Patent Application No. 2009/0122568, which teaches the application of white light emitting LEDs and colored filters to provide the necessary amber illumination of turn signal indicators.
The prior art thus fails to provide turn signal indicators presenting a first color when not illuminated and a second, necessary color when they are illuminated.